Gentamicin (120 mg/kg/day) was administered for 10 days to Sprague-Dawley rats given either a low (5% w/w) or normal (18% w/w) protein diet. Serum protein concentrations remained normal in all rats during the study. Nephrotoxicity was slightly less severe in rats fed a low protein diet as shown by: (i) a mean creatinine clearance rate (14 +/- 4 ml/min) which was significantly greater than that (8 +/- 3 ml/min) recorded from the rats maintained on the normal diet (P less than 0.05); (ii) lower activities of urinary N-acetyl-beta-D glucosaminidase (NAG); and (iii) less marked histological changes. Mean tissue concentrations of gentamicin were considerably lower in both renal cortex and medulla from rats maintained on the low protein diet than from those animals on the normal diet (P less than 0.01 and P less than 0.05, respectively). These differences were, however, not reflected in the mean trough serum gentamicin concentrations which were not significantly different between the two groups. These results are discussed in relation to the proposed mechanisms involved in gentamicin-induced nephrotoxicity.